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linux, crontab

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1. Crontab Restrictions
____________
You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file /usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.


2. Crontab Commands
__________
export EDITOR=vi ;to specify a editor to open crontab file.

crontab -e     Edit your crontab file, or create one if it doesn’t already exist.
crontab -l      Display your crontab file.
crontab -r      Remove your crontab file.
crontab -v      Display the last time you edited your crontab file. (This option is only available on a few systems.)

3. Crontab file
___________
Crontab syntax :-
A crontab file has five fields for specifying day , date and time  followed by the command to be run at that interval.

*     *   *   *    *  command to be executed
–     –    –    –    –
|     |     |     |     |
|     |     |     |     +—– day of week (0 – 6) (Sunday=0)
|     |     |     +——- month (1 – 12)
|     |     +——— day of month (1 – 31)
|     +———– hour (0 – 23)
+————- min (0 – 59)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

Note: The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed .

4. Crontab Example
_______

A line in crontab file like below  removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30     18     *     *     *         rm /home/someuser/tmp/*

Changing the parameter values as below will cause this command to run at different time schedule below :

min hour day/month month day/week Execution time
30 0 1 1,6,12 * — 00:30 Hrs  on 1st of Jan, June & Dec.
:
0 20 * 10 1-5 –8.00 PM every weekday (Mon-Fri) only in Oct.
:
0 0 1,10,15 * * — midnight on 1st ,10th & 15th of month
:
5,10 0 10 * 1 — At 12.05,12.10 every Monday & on 10th of every month
:

Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file

rianto utomo bole nyontek dari http://www.adminschoice.com/docs/crontab.htm

Written by isaaconi

Desember 15, 2009 pada 3:02 pm

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